Accurate skew determination for magnetic tapes experiencing the effects of tape dimensional instability

ABSTRACT

A computer-implemented method, according to one embodiment, includes: receiving a first timestamp in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape, and receiving a second timestamp in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape. A third timestamp is received in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape, while a fourth timestamp is received in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape. The first, second, third, and fourth timestamps are used to determine the skew of the magnetic tape. Further still, the first and third servo bursts are in a same first servo sub-frame, while the second and fourth servo bursts are in a same second servo sub-frame.

BACKGROUND

The present invention relates to data storage systems, and more particularly, this invention relates to tape-based data storage systems having the ability to determine skew for magnetic tapes despite experiencing tape dimensional stability issues.

In magnetic storage systems, magnetic transducers read data from and write data onto magnetic recording media. Data is written on the magnetic recording media by moving a magnetic recording transducer to a position over the media where the data is to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic media. Data is read from the media by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic media. Read and write operations may be independently synchronized with the movement of the media to ensure that the data can be read from and written to the desired location on the media.

An important and continuing goal in the data storage industry is that of increasing the density of data stored on a medium. For tape storage systems, that goal has led to increasing the track and linear bit density on recording tape, and decreasing the thickness of the magnetic tape medium. However, the development of small footprint, higher performance tape drive systems has created various problems in the design of a tape head assembly for use in such systems.

In a tape drive system, the drive moves the magnetic tape over the surface of the tape head at high speed. Usually the tape head is designed to minimize the spacing between the head and the tape. The spacing between the magnetic head and the magnetic tape is crucial and so goals in these systems are to have the recording gaps of the transducers, which are the source of the magnetic recording flux in near contact with the tape to effect writing sharp transitions, and to have the read elements in near contact with the tape to provide effective coupling of the magnetic field from the tape to the read elements.

The quantity of data stored on a magnetic tape may be expanded by increasing the number of data tracks across the tape. More tracks are made possible by reducing feature sizes of the readers and writers, such as by using thin-film fabrication techniques and magnetoresistive (MR) sensors. However, for various reasons, the feature sizes of readers and writers cannot be arbitrarily reduced, and so factors such as tape skew, lateral tape motion (e.g., perpendicular to the direction of tape travel), transients and tape lateral expansion and contraction must be balanced with reader/writer sizes that provide acceptable written tracks and readback signals.

As alluded to in the previous paragraph, one issue limiting areal density is misregistration caused by tape skew. As tape is being run over the surface of a tape head, angular shifts in the relative angular relationship between an array of transducers and the data tracks on the tape may occur, typically resulting in misregistration of the transducers from the data tracks of the tape. Skew-induced misregistration tends to be more pronounced on smaller tracks. Accordingly, data may not be readable from a tape, particularly as the tape capacity increases over time and tracks become smaller.

Another particular problem limiting areal density is misregistration caused by tape lateral expansion and contraction, commonly referred to as poor tape dimensional stability (TDS), or more properly, tape dimensional instability (TDI). Tape lateral contraction and expansion is a well-known phenomenon that occurs due to a plethora of effects, including absorption of water, thermal expansion and contraction, etc. Tape width can vary by up to about 0.1% due to TDS/TDI.

When the dimensions of the tape change, various issues arise. During writing, the likelihood of overwriting shingled tracks increases, where overwritten data is often unrecoverable. Likewise, where the width of the tape has changed since the desired data was written, the readers may no longer be positioned over the tracks to be read, increasing reading errors. The extent of misregistration is particularly prevalent toward outer ends of the reader array.

More permanent changes in media lateral dimensions may also occur, such as long-term media “creep” (also known in the art as “aging”), which tends to occur over time when a tape is wound around a hub of a tape cartridge. Long-term media creep is particularly problematic when dealing with tape dimensional stability issues, as the two ends of the tape exhibit creep in different ways. The inner wraps of tape positioned closest to the cartridge hub tend to expand laterally over time due to the compressive stresses exerted thereon by the wraps of tape wound around them. Wraps positioned toward the outer diameter of the spool of tape are under less compressive stress, but are under higher tensile stresses, which tends to cause lateral contraction of the tape, i.e., the tape becomes narrower over time. Accordingly, the ends of the tape tend to exhibit oppositely-oriented lateral dimensional changes. It follows that shingled track pitch changes over time due to creep, and to different extents (non-uniformly) along the length of the tape.

Thus, while the width of the transducer array is essentially fixed (absent tilting or other mechanism to adjust the transducer spacing presented to tape), the spacing of the data tracks on the tape will vary as the tape expands and contracts. Ideally, the reader track width would be as wide as the data track being read; this would provide the best signal. However, sensor track widths cannot be made as wide as the data tracks, because the sensors would read adjacent tracks upon expansion or contraction of the tape and/or due to lateral misregistration between tape and head. Accordingly, reader widths are currently designed to be substantially smaller than the data track width, and all readers in a given head have the same track width. The reader track width is selected to accommodate the worst-case scenarios, i.e., the designer takes into account maximum expansion/contraction and lateral misregistration when determining reader track width so that each sensor is over a given track at any time.

What is needed is a solution to the foregoing problems, thereby enabling future areal density growth.

SUMMARY

The apparatuses and methods presented herein address difficulties encountered when attempting to increase the storage density on magnetic tape media while maintaining a high data rate per unit of tape speed.

A computer-implemented method, according to one embodiment, is for determining skew of a magnetic tape in a TDS environment. The computer-implemented method includes: receiving a first timestamp t¹ _(B) in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape, and receiving a second timestamp t² _(B) in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape. A third timestamp t¹ _(A) is also received in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape, while a fourth timestamp t² _(A) is received in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape. Moreover, the first, second, third, and fourth timestamps are used to determine the skew of the magnetic tape. Further still, the first and third servo bursts are in a same first servo sub-frame, while the second and fourth servo bursts are in a same second servo sub-frame.

A computer program product, according to another embodiment, is for determining skew of a magnetic tape in a TDS environment. The computer program product includes a computer readable storage medium having program instructions embodied therewith. The program instructions are readable and/or executable by a processor to cause the processor to: perform the foregoing method.

A system, according to yet another embodiment, includes: a tape drive having at least two servo readers, a processor, and logic that is integrated with the processor, executable by the processor, or integrated with and executable by the processor. Moreover, the logic is configured to: perform the foregoing method.

Any of these approaches may be implemented in a magnetic data storage system such as a tape drive system, which may include a magnetic head, a drive mechanism for passing a magnetic medium (e.g., recording tape) over the magnetic head, and a controller electrically coupled to the magnetic head.

Other aspects and approaches of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a simplified tape drive system, in accordance with one approach.

FIG. 1B is a schematic diagram of a tape cartridge, in accordance with one approach.

FIG. 2A is a side view of a flat-lapped, bi-directional, two-module magnetic tape head, in accordance with one approach.

FIG. 2B is a tape bearing surface view taken from Line 2B of FIG. 2A.

FIG. 2C is a detailed view taken from Circle 2C of FIG. 2B.

FIG. 2D is a detailed view of a partial tape bearing surface of a pair of modules.

FIG. 3 is a partial tape bearing surface view of a magnetic head having a write-read-write configuration.

FIG. 4 is a partial tape bearing surface view of a magnetic head having a read-write-read configuration.

FIG. 5 is a side view of a magnetic tape head with three modules according to one approach where the modules all generally lie along about parallel planes.

FIG. 6 is a side view of a magnetic tape head with three modules in a tangent (angled) configuration.

FIG. 7 is a side view of a magnetic tape head with three modules in an overwrap configuration.

FIGS. 8A-8C are schematics depicting the principles of tape tenting.

FIG. 9 is a representational diagram of files and indexes stored on a magnetic tape, in accordance with one approach.

FIG. 10 is a tape layout, in accordance with one aspect of the present invention.

FIG. 11 is a representational view of a hybrid servo pattern written in a dedicated area of a tape medium, in accordance with one aspect of the present invention.

FIG. 12 is a partial detailed view of a (TBS) pattern, in accordance with one aspect of the present invention.

FIG. 13 is a graph plotting sample vs. amplitude of the TBS pattern of FIG. 12, in accordance with one aspect of the present invention.

FIG. 14 is a block diagram of a detector for servo patterns, in accordance with one approach.

FIGS. 15A-15D are representative drawings showing the effect of tape skew on transducer positions relative thereto.

FIGS. 16A-16C illustrate the effect of tape lateral contraction.

FIGS. 17A-17D illustrate the effect of skew paired with tape lateral contraction.

FIGS. 18A-18B are partial representational views of magnetic tapes positioned in tape drives, in accordance with two approaches.

FIG. 18C is a partial representational view of a servo frame, in accordance with one approach.

FIG. 19 is a flowchart of a method, in accordance with one approach.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.

The following description discloses several preferred approaches of magnetic storage systems, as well as operation and/or component parts thereof.

In one general embodiment, a computer-implemented method is for determining skew of a magnetic tape in a TDS environment. The computer-implemented method includes: receiving a first timestamp t¹ _(B) in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape, and receiving a second timestamp t² _(B) in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape. A third timestamp t¹ _(A) is also received in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape, while a fourth timestamp t² _(A) is received in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape. Moreover, the first, second, third, and fourth timestamps are used to determine the skew of the magnetic tape. Further still, the first and third servo bursts are in a same first servo sub-frame, while the second and fourth servo bursts are in a same second servo sub-frame.

In another general embodiment, a computer program product is for determining skew of a magnetic tape in a TDS environment. The computer program product includes a computer readable storage medium having program instructions embodied therewith. The program instructions are readable and/or executable by a processor to cause the processor to: perform the foregoing method.

In yet another general embodiment, a system includes: a tape drive having at least two servo readers, a processor, and logic that is integrated with the processor, executable by the processor, or integrated with and executable by the processor. Moreover, the logic is configured to: perform the foregoing method.

FIG. 1A illustrates a simplified tape drive 100 of a tape-based data storage system, which may be employed in the context of the present invention. While one specific implementation of a tape drive is shown in FIG. 1A, it should be noted that the approaches described herein may be implemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 are provided to support a tape 122. One or more of the reels may form part of a removable cartridge and are not necessarily part of the tape drive 100. The tape drive, such as that illustrated in FIG. 1A, may further include drive motor(s) to drive the tape supply cartridge 120 and the take-up reel 121 to move the tape 122 over a tape head 126 of any type. Such head may include an array of readers, writers, or both.

Guides 125 guide the tape 122 across the tape head 126. Such tape head 126 is in turn coupled to a controller 128 via a cable 130. The controller 128, may be or include a processor and/or any logic for controlling any subsystem of the drive 100. For example, the controller 128 typically controls head functions such as servo following, data writing, data reading, etc. The controller 128 may include at least one servo channel and at least one data channel, each of which include data flow processing logic configured to process and/or store information to be written to and/or read from the tape 122. The controller 128 may operate under logic known in the art, as well as any logic disclosed herein, and thus may be considered as a processor for any of the descriptions of tape drives included herein, in various approaches. The controller 128 may be coupled to a memory 136 of any known type, which may store instructions executable by the controller 128. Moreover, the controller 128 may be configured and/or programmable to perform or control some or all of the methodology presented herein. Thus, the controller 128 may be considered to be configured to perform various operations by way of logic programmed into one or more chips, modules, and/or blocks; software, firmware, and/or other instructions being available to one or more processors; etc., and combinations thereof.

The cable 130 may include read/write circuits to transmit data to the tape head 126 to be recorded on the tape 122 and to receive data read by the tape head 126 from the tape 122. An actuator 132 controls position of the tape head 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tape drive 100 and a host (internal or external) to send and receive the data and for controlling the operation of the tape drive 100 and communicating the status of the tape drive 100 to the host, all as will be understood by those of skill in the art.

FIG. 1B illustrates an exemplary tape cartridge 150 according to one approach. Such tape cartridge 150 may be used with a system such as that shown in FIG. 1A. As shown, the tape cartridge 150 includes a housing 152, a tape 122 in the housing 152, and a nonvolatile memory 156 coupled to the housing 152. In some approaches, the nonvolatile memory 156 may be embedded inside the housing 152, as shown in FIG. 1B. In more approaches, the nonvolatile memory 156 may be attached to the inside or outside of the housing 152 without modification of the housing 152. For example, the nonvolatile memory may be embedded in a self-adhesive label 154. In one preferred approach, the nonvolatile memory 156 may be a Flash memory device, read-only memory (ROM) device, etc., embedded into or coupled to the inside or outside of the tape cartridge 150. The nonvolatile memory is accessible by the tape drive and the tape operating software (the driver software), and/or another device.

By way of example, FIG. 2A illustrates a side view of a flat-lapped, bi-directional, two-module magnetic tape head 200 which may be implemented in the context of the present invention. As shown, the head includes a pair of bases 202, each equipped with a module 204, and fixed at a small angle α with respect to each other. The bases may be “U-beams” that are adhesively coupled together. Each module 204 includes a substrate 204A and a closure 204B with a thin film portion, commonly referred to as a “gap” in which the readers and/or writers 206 are formed. In use, a tape 208 is moved over the modules 204 along a media (tape) bearing surface 209 in the manner shown for reading and writing data on the tape 208 using the readers and writers. The wrap angle θ of the tape 208 at edges going onto and exiting the flat media support surfaces 209 are usually between about 0.1 degree and about 3 degrees.

The substrates 204A are typically constructed of a wear resistant material, such as a ceramic. The closures 204B may be made of the same or similar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback or merged configuration. An illustrative piggybacked configuration comprises a (magnetically inductive) writer transducer on top of (or below) a (magnetically shielded) reader transducer (e.g., a magnetoresistive reader, etc.), wherein the poles of the writer and the shields of the reader are generally separated. An illustrative merged configuration comprises one reader shield in the same physical layer as one writer pole (hence, “merged”). The readers and writers may also be arranged in an interleaved configuration. Alternatively, each array of channels may be readers or writers only. Any of these arrays may contain one or more servo readers for reading servo data on the medium.

FIG. 2B illustrates the tape bearing surface 209 of one of the modules 204 taken from Line 2B of FIG. 2A. A representative tape 208 is shown in dashed lines. The module 204 is preferably long enough to be able to support the tape as the head steps between data bands.

In this example, the tape 208 includes 4 to 32 data bands, e.g., with 16 data bands and 17 servo bands 210, as shown in FIG. 2B on a one-half inch wide tape 208. The data bands are defined between servo bands 210. Each data band may include a number of data tracks, for example 1024 data tracks (not shown). During read/write operations, the readers and/or writers 206 are positioned to specific track positions within one of the data bands. Outer readers, sometimes called servo readers, read the servo tracks in the servo bands 210. The servo signals are in turn used for track following, i.e., to keep the readers and/or writers 206 aligned with a particular set of tracks during the read/write operations.

FIG. 2C depicts a plurality of readers and/or writers 206 formed in a gap 218 on the module 204 in Circle 2C of FIG. 2B. As shown, the array of readers and writers 206 includes, for example, 16 writers 214, 16 readers 216 and two servo readers 212, though the number of elements may vary. Illustrative approaches include 8, 16, 32, 40, and 64 active readers and/or writers 206 per array, and alternatively interleaved designs having odd numbers of reader or writers such as 17, 25, 33, etc. An illustrative approach includes 32 readers per array and/or 32 writers per array, where the actual number of transducer elements could be greater, e.g., 33, 34, etc. This allows the tape to travel more slowly, thereby reducing speed-induced tracking and mechanical difficulties and/or execute fewer “wraps” to fill or read the tape. While the readers and writers may be arranged in a piggyback configuration as shown in FIG. 2C, the readers 216 and writers 214 may also be arranged in an interleaved configuration. Alternatively, each array of readers and/or writers 206 may be readers or writers only, and the arrays may contain one or more servo readers 212. As noted by considering FIGS. 2A and 2B-2C together, each module 204 may include a complementary set of readers and/or writers 206 for such things as bi-directional reading and writing, read-while-write capability, backward compatibility, etc.

FIG. 2D shows a partial tape bearing surface view of complementary modules of a magnetic tape head 200 according to one approach. In this approach, each module has a plurality of read/write (R/W) pairs in a piggyback configuration formed on a common substrate 204A and an optional electrically insulative insulating layer 236. The writers 214 and the readers 216 are aligned parallel to an intended direction of travel of a tape medium thereacross to form an RAY pair, exemplified by RAY pairs 222. Note that the intended direction of tape travel is sometimes referred to herein as the direction of tape travel, and such terms may be used interchangeably. Such direction of tape travel may be inferred from the design of the system, e.g., by examining the guides; observing the actual direction of tape travel relative to the reference point; etc. Moreover, in a system operable for bi-direction reading and/or writing, the direction of tape travel in both directions is typically parallel and thus both directions may be considered equivalent to each other.

Several R/W pairs 222 may be present, such as 8, 16, 32 pairs, etc. The R/W pairs 222 as shown are linearly aligned in a direction generally perpendicular to a direction of tape travel thereacross. However, the pairs may also be aligned diagonally, etc. Servo readers 212 are positioned on the outside of the array of RAY pairs, the function of which is well known.

Generally, the magnetic tape medium moves in either a forward or reverse direction as indicated by arrow 220. The magnetic tape medium and head assembly 200 operate in a transducing relationship in the manner well-known in the art. The head assembly 200 includes two thin-film modules 224 and 226 of generally identical construction.

Modules 224 and 226 are joined together with a space present between closures 204B thereof (partially shown) to form a single physical unit to provide read-while-write capability by activating the writer of the leading module and reader of the trailing module aligned with the writer of the leading module parallel to the direction of tape travel relative thereto. When a module 224, 226 of a magnetic tape head 200 is constructed, layers are formed in the gap 218 created above an electrically conductive substrate 204A (partially shown), e.g., of AlTiC, in generally the following order for the R/W pairs 222: an insulating layer 236, a first shield 232 typically of an iron alloy such as NiFe (—), cobalt zirconium tantalum (CZT) or Al—Fe—Si (Sendust), a sensor 234 for sensing a data track on a magnetic medium, a second shield 238 typically of a nickel-iron alloy (e.g., ˜80/20 at % NiFe, also known as permalloy), first and second writer poles 228, 230, and a coil (not shown). The sensor may be of any known type, including those based on magnetoresistive (MR), GMR, AMR, tunneling magnetoresistance (TMR), etc.

The first and second writer poles 228, 230 may be fabricated from high magnetic moment materials such as ˜45/55 NiFe. Note that these materials are provided by way of example only, and other materials may be used. Additional layers such as insulation between the shields and/or pole tips and an insulation layer surrounding the sensor may be present. Illustrative materials for the insulation include alumina and other oxides, insulative polymers, etc.

The configuration of the tape head 126 according to one approach includes multiple modules, preferably three or more. In a write-read-write (W-R-W) head, outer modules for writing flank one or more inner modules for reading. Referring to FIG. 3, depicting a W-R-W configuration, the outer modules 252, 256 each include one or more arrays of writers 260. The inner module 254 of FIG. 3 includes one or more arrays of readers 258 in a similar configuration. Variations of a multi-module head include a R-W-R head (FIG. 4), a R-R-W head, a W-W-R head, etc. In yet other variations, one or more of the modules may have read/write pairs of transducers. Moreover, more than three modules may be present. In further approaches, two outer modules may flank two or more inner modules, e.g., in a W-R-R-W, a R-W-W-R arrangement, etc. For simplicity, a W-R-W head is used primarily herein to exemplify approaches of the present invention. One skilled in the art apprised with the teachings herein will appreciate how permutations of the present invention would apply to configurations other than a W-R-W configuration.

FIG. 5 illustrates a magnetic head 126 according to one approach of the present invention that includes first, second and third modules 302, 304, 306 each having a tape bearing surface 308, 310, 312 respectively, which may be flat, contoured, etc. Note that while the term “tape bearing surface” appears to imply that the surface facing the tape 315 is in physical contact with the tape bearing surface, this is not necessarily the case. Rather, only a portion of the tape may be in contact with the tape bearing surface, constantly or intermittently, with other portions of the tape riding (or “flying”) above the tape bearing surface on a layer of air, sometimes referred to as an “air bearing”. The first module 302 will be referred to as the “leading” module as it is the first module encountered by the tape in a three-module design for tape moving in the indicated direction. The third module 306 will be referred to as the “trailing” module. The trailing module follows the middle module and is the last module seen by the tape in a three-module design. The leading and trailing modules 302, 306 are referred to collectively as outer modules. Also note that the outer modules 302, 306 will alternate as leading modules, depending on the direction of travel of the tape 315.

In one approach, the tape bearing surfaces 308, 310, 312 of the first, second and third modules 302, 304, 306 lie on about parallel planes (which is meant to include parallel and nearly parallel planes, e.g., between parallel and tangential as in FIG. 6), and the tape bearing surface 310 of the second module 304 is above the tape bearing surfaces 308, 312 of the first and third modules 302, 306. As described below, this has the effect of creating the desired wrap angle α₂ of the tape relative to the tape bearing surface 310 of the second module 304.

Where the tape bearing surfaces 308, 310, 312 lie along parallel or nearly parallel yet offset planes, intuitively, the tape should peel off of the tape bearing surface 308 of the leading module 302. However, the vacuum created by a skiving edge 318 of the leading module 302 has been found by experimentation to be sufficient to keep the tape adhered to the tape bearing surface 308 of the leading module 302. A trailing edge 320 of the leading module 302 (the end from which the tape leaves the leading module 302) is the approximate reference point which defines the wrap angle α2 over the tape bearing surface 310 of the second module 304. The tape stays in close proximity to the tape bearing surface until close to the trailing edge 320 of the leading module 302. Accordingly, transducers 322 may be located near the trailing edges of the outer modules 302, 306. These approaches are particularly adapted for write-read-write applications.

A benefit of this and other approaches described herein is that, because the outer modules 302, 306 are fixed at a determined offset from the second module 304, the inner wrap angle α₂ is fixed when the modules 302, 304, 306 are coupled together or are otherwise fixed into a head. The inner wrap angle α₂ is approximately tan⁻¹ (δ/W) where δ is the height difference between the planes of the tape bearing surfaces 308, 310 and W is the width between the opposing ends of the tape bearing surfaces 308, 310. An illustrative inner wrap angle α₂ is in a range of about 0.3° to about 1.1°, though can be any angle required by the design.

Beneficially, the inner wrap angle α₂ on the side of the module 304 receiving the tape (leading edge) will be larger than the inner wrap angle α₃ on the trailing edge, as the tape 315 rides above the trailing module 306. This difference is generally beneficial as a smaller α₃ tends to oppose what has heretofore been a steeper exiting effective wrap angle.

Note that the tape bearing surfaces 308, 312 of the outer modules 302, 306 are positioned to achieve a negative wrap angle at the trailing edge 320 of the leading module 302. This is generally beneficial in helping to reduce friction due to contact with the trailing edge 320, provided that proper consideration is given to the location of the crowbar region that forms in the tape where it peels off the head. This negative wrap angle also reduces flutter and scrubbing damage to the elements on the leading module 302. Further, at the trailing module 306, the tape 315 flies over the tape bearing surface 312 so there is virtually no wear on the elements when tape is moving in this direction. Particularly, the tape 315 entrains air and so will not significantly ride on the tape bearing surface 312 of the third module 306 (some contact may occur). This is permissible, because the leading module 302 is writing while the trailing module 306 is idle.

Writing and reading functions are performed by different modules at any given time. In one approach, the second module 304 includes a plurality of data and optional servo readers 331 and no writers. The first and third modules 302, 306 include a plurality of writers 322 and no data readers, with the exception that the outer modules 302, 306 may include optional servo readers. The servo readers may be used to position the head during reading and/or writing operations. The servo reader(s) on each module are typically located towards the end of the array of readers or writers.

By having only readers or side by side writers and servo readers in the gap between the substrate and closure, the gap length can be substantially reduced. Typical heads have piggybacked readers and writers, where the writer is formed above each reader. A typical gap is 20-35 microns. However, irregularities on the tape may tend to droop into the gap and create gap erosion. Thus, the smaller the gap is the better. The smaller gap enabled herein exhibits fewer wear related problems.

In some aspects, the second module 304 has a closure, while the first and third modules 302, 306 do not have a closure. Where there is no closure, preferably a hard coating is added to the module. One preferred coating is diamond-like carbon (DLC).

In the approach shown in FIG. 5, the first, second, and third modules 302, 304, 306 each have a closure 332, 334, 336, which extends the tape bearing surface of the associated module, thereby effectively positioning the read/write elements away from the edge of the tape bearing surface. The closure 332 on the second module 304 can be a ceramic closure of a type typically found on tape heads. The closures 334, 336 of the first and third modules 302, 306, however, may be shorter than the closure 332 of the second module 304 as measured parallel to a direction of tape travel over the respective module. This enables positioning the modules closer together. One way to produce shorter closures 334, 336 is to lap the standard ceramic closures of the second module 304 an additional amount. Another way is to plate or deposit thin film closures above the elements during thin film processing. For example, a thin film closure of a hard material such as Sendust or nickel-iron alloy (e.g., 45/55) can be formed on the module.

With reduced-thickness ceramic or thin film closures 334, 336 or no closures on the outer modules 302, 306, the write-to-read gap spacing can be reduced to less than about 1 mm, e.g., about 0.75 mm, or 50% less than commonly-used linear tape open (LTO) tape head spacing. The open space between the modules 302, 304, 306 can still be set to approximately 0.5 to 0.6 mm, which in some approaches is ideal for stabilizing tape motion over the second module 304.

Depending on tape tension and stiffness, it may be desirable to angle the tape bearing surfaces of the outer modules relative to the tape bearing surface of the second module. FIG. 6 illustrates an approach where the modules 302, 304, 306 are in a tangent or nearly tangent (angled) configuration. Particularly, the tape bearing surfaces of the outer modules 302, 306 are about parallel to the tape at the desired wrap angle α₂ of the second module 304. In other words, the planes of the tape bearing surfaces 308, 312 of the outer modules 302, 306 are oriented at about the desired wrap angle α₂ of the tape 315 relative to the second module 304. The tape will also pop off of the trailing module 306 in this approach, thereby reducing wear on the elements in the trailing module 306. These approaches are particularly useful for write-read-write applications. Additional aspects of these approaches are similar to those given above.

Typically, the tape wrap angles may be set about midway between the approaches shown in FIGS. 5 and 6.

FIG. 7 illustrates an approach where the modules 302, 304, 306 are in an overwrap configuration. Particularly, the tape bearing surfaces 308, 312 of the outer modules 302, 306 are angled slightly more than the tape 315 when set at the desired wrap angle α₂ relative to the second module 304. In this approach, the tape does not pop off of the trailing module, allowing it to be used for writing or reading. Accordingly, the leading and middle modules can both perform reading and/or writing functions while the trailing module can read any just-written data. Thus, these approaches are preferred for write-read-write, read-write-read, and write-write-read applications. In the latter approaches, closures should be wider than the tape canopies for ensuring read capability. The wider closures may require a wider gap-to-gap separation. Therefore, a preferred approach has a write-read-write configuration, which may use shortened closures that thus allow closer gap-to-gap separation.

Additional aspects of the approaches shown in FIGS. 6 and 7 are similar to those given above.

A 32 channel version of a multi-module tape head 126 may use cables 350 having leads on the same or smaller pitch as current 16 channel piggyback LTO modules, or alternatively the connections on the module may be organ-keyboarded for a 50% reduction in cable span. Over-under, writing pair unshielded cables may be used for the writers, which may have integrated servo readers.

The outer wrap angles α₁ may be set in the drive, such as by guides of any type known in the art, such as adjustable rollers, slides, etc. or alternatively by outriggers, which are integral to the head. For example, rollers having an offset axis may be used to set the wrap angles. The offset axis creates an orbital arc of rotation, allowing precise alignment of the wrap angle α₁.

To assemble any of the approaches described above, conventional u-beam assembly can be used. Accordingly, the mass of the resultant head may be maintained or even reduced relative to heads of previous generations. In other approaches, the modules may be constructed as a unitary body. Those skilled in the art, armed with the present teachings, will appreciate that other known methods of manufacturing such heads may be adapted for use in constructing such heads. Moreover, unless otherwise specified, processes and materials of types known in the art may be adapted for use in various approaches in conformance with the teachings herein, as would become apparent to one skilled in the art upon reading the present disclosure.

As a tape is run over a module, it is preferred that the tape passes sufficiently close to magnetic transducers on the module such that reading and/or writing is efficiently performed, e.g., with a low error rate. According to some approaches, tape tenting may be used to ensure the tape passes sufficiently close to the portion of the module having the magnetic transducers. To better understand this process, FIGS. 8A-8C illustrate the principles of tape tenting. FIG. 8A shows a module 800 having an upper tape bearing surface 802 extending between opposite edges 804, 806. A stationary tape 808 is shown wrapping around the edges 804, 806. As shown, the bending stiffness of the tape 808 lifts the tape off of the tape bearing surface 802. Tape tension tends to flatten the tape profile, as shown in FIG. 8A. Where tape tension is minimal, the curvature of the tape is more parabolic than shown.

FIG. 8B depicts the tape 808 in motion. The leading edge, i.e., the first edge the tape encounters when moving, may serve to skive air from the tape, thereby creating a subambient air pressure between the tape 808 and the tape bearing surface 802. In FIG. 8B, the leading edge is the left edge and the right edge is the trailing edge when the tape is moving left to right. As a result, atmospheric pressure above the tape urges the tape toward the tape bearing surface 802, thereby creating tape tenting proximate each of the edges. The tape bending stiffness resists the effect of the atmospheric pressure, thereby causing the tape tenting proximate both the leading and trailing edges. Modeling predicts that the two tents are very similar in shape.

FIG. 8C depicts how the subambient pressure urges the tape 808 toward the tape bearing surface 802 even when a trailing guide 810 is positioned above the plane of the tape bearing surface.

It follows that tape tenting may be used to direct the path of a tape as it passes over a module. As previously mentioned, tape tenting may be used to ensure the tape passes sufficiently close to the portion of the module having the magnetic transducers, preferably such that reading and/or writing is efficiently performed, e.g., with a low error rate.

Magnetic tapes may be stored in tape cartridges that are, in turn, stored at storage slots or the like inside a data storage library. The tape cartridges may be stored in the library such that they are accessible for physical retrieval. In addition to magnetic tapes and tape cartridges, data storage libraries may include data storage drives that store data to, and/or retrieve data from, the magnetic tapes. Moreover, tape libraries and the components included therein may implement a file system which enables access to tape and data stored on the tape.

File systems may be used to control how data is stored in, and retrieved from, memory. Thus, a file system may include the processes and data structures that an operating system uses to keep track of files in memory, e.g., the way the files are organized in memory. Linear Tape File System (LTFS) is an exemplary format of a file system that may be implemented in a given library in order to enables access to compliant tapes. It should be appreciated that various approaches herein can be implemented with a wide range of file system formats, including for example IBM Spectrum Archive Library Edition (LTFS LE). However, to provide a context, and solely to assist the reader, some of the approaches below may be described with reference to LTFS which is a type of file system format. This has been done by way of example only, and should not be deemed limiting on the invention defined in the claims.

A tape cartridge may be “loaded” by inserting the cartridge into the tape drive, and the tape cartridge may be “unloaded” by removing the tape cartridge from the tape drive. Once loaded in a tape drive, the tape in the cartridge may be “threaded” through the drive by physically pulling the tape (the magnetic recording portion) from the tape cartridge, and passing it above a magnetic head of a tape drive. Furthermore, the tape may be attached on a take-up reel (e.g., see 121 of FIG. 1A above) to move the tape over the magnetic head.

Once threaded in the tape drive, the tape in the cartridge may be “mounted” by reading metadata on a tape and bringing the tape into a state where the LTFS is able to use the tape as a constituent component of a file system. Moreover, in order to “unmount” a tape, metadata is preferably first written on the tape (e.g., as an index), after which the tape may be removed from the state where the LTFS is allowed to use the tape as a constituent component of a file system. Finally, to “unthread” the tape, the tape is unattached from the take-up reel and is physically placed back into the inside of a tape cartridge again. The cartridge may remain loaded in the tape drive even after the tape has been unthreaded, e.g., waiting for another read and/or write request. However, in other instances, the tape cartridge may be unloaded from the tape drive upon the tape being unthreaded, e.g., as described above.

Magnetic tape is a sequential access medium. Thus, new data is written to the tape by appending the data at the end of previously written data. It follows that when data is recorded in a tape having only one partition, metadata (e.g., allocation information) is continuously appended to an end of the previously written data as it frequently updates and is accordingly rewritten to tape. As a result, the rearmost information is read when a tape is first mounted in order to access the most recent copy of the metadata corresponding to the tape. However, this introduces a considerable amount of delay in the process of mounting a given tape.

To overcome this delay caused by single partition tape mediums, the LTFS format includes a tape that is divided into two partitions, which include an index partition and a data partition. The index partition may be configured to record metadata (meta information), e.g., such as file allocation information (Index), while the data partition may be configured to record the body of the data, e.g., the data itself.

Looking to FIG. 9, a magnetic tape 900 having an index partition 902 and a data partition 904 is illustrated according to one approach. As shown, data files and indexes are stored on the tape. The LTFS format allows for index information to be recorded in the index partition 902 at the beginning of tape 906, as would be appreciated by one skilled in the art upon reading the present description.

As index information is updated, it preferably overwrites the previous version of the index information, thereby allowing the currently updated index information to be accessible at the beginning of tape in the index partition. According to the specific example illustrated in FIG. 9, a most recent version of metadata Index 3 is recorded in the index partition 902 at the beginning of the tape 906. Conversely, all three version of metadata Index 1, Index 2, Index 3 as well as data File A, File B, File C, File D are recorded in the data partition 904 of the tape. Although Index 1 and Index 2 are old (e.g., outdated) indexes, because information is written to tape by appending it to the end of the previously written data as described above, these old indexes Index 1, Index 2 remain stored on the tape 900 in the data partition 904 without being overwritten.

The metadata may be updated in the index partition 902 and/or the data partition 904 the same or differently depending on the desired approach. According to some approaches, the metadata of the index and/or data partitions 902, 904 may be updated in response to the tape being unmounted, e.g., such that the index may be read quickly from the index partition when that tape is mounted again. The metadata is preferably also written in the data partition 904 so the tape may be mounted using the metadata recorded in the data partition 904, e.g., as a backup option.

According to one example, which is no way intended to limit the invention, LTFS LE may be used to provide the functionality of writing an index in the data partition when a user explicitly instructs the system to do so, or at a time designated by a predetermined period which may be set by the user, e.g., such that data loss in the event of sudden power stoppage can be mitigated.

Referring to FIG. 10, an illustrative tape layout is depicted in accordance with one aspect. As shown, a magnetic recording tape 1000 has a tape layout which implements five servo bands Servo Band 0-Servo Band 4, and four data bands Data Band 0-Data Band 3, e.g., as specified in the LTO format and IBM® Enterprise format. The height H of each of the timing-based servo (TBS) servo bands is measured in the cross-track direction 1004 which is about orthogonal to the length L of the tape 1000. According to an example, the height H of each of the servo bands may be about 186 microns according to the LTO format. Moreover, a pitch β between the servo bands as shown may be about 2859 microns, again according to the LTO format.

An exemplary tape head 1002 is also shown as having two modules and as being positioned over a portion of the tape 1000 according to one approach. Read and/or write transducers may be positioned on either module of the tape head 1002 according to any of the approaches described herein, and may be used to read data from and/or write data to the data bands. Furthermore, tape head 1002 may include servo readers which may be used to read the servo patterns in the servo bands according to any of the approaches described herein. It should also be noted that the dimensions of the various components included in FIG. 10 are presented by way of example only and are in no way intended to be limiting.

Some tape drives may be configured to operate at low tape velocities and/or with nanometer head position settings. These tape drives may use servo formats that target Barium Ferrite (BaFe) tape media, 4 or 8 data bands, 32 or 64 data channel operation, allow very low velocity operation, support large-bandwidth actuator operation, and improve parameter estimation to minimize standard deviation of the position error signal (PES), thus enabling track-density scaling for tape cartridge capacities up to 100 TB and beyond.

However, according to some approaches, magnetic tape may further be augmented with additional features that provide additional functionality. Accordingly, HD servo patterns may be implemented with standard TBS patterns. The HD servo patterns may be used to improve track-following performance. Accordingly, a standard TBS pattern (e.g., as shown in FIG. 10) may be implemented in combination with one or more HD servo patterns. One implementation includes a servo pattern scheme in which a standard TBS pattern is retained and additional HD patterns are provided in a dedicated, preferably currently unused area of the tape media. This type of pattern may be implemented by increasing the number of data channels from 16 to 32, and reducing the width of the TBS pattern from 186 microns to 93 microns, in some approaches.

FIG. 11 depicts a representation of a data band 1100 having a hybrid servo pattern 1110, which includes a TBS pattern 1102 written in a servo band, as well as an HD pattern 1104 that is written in a HD band (e.g., dedicated area) of the tape medium 1108. Moreover, each HD pattern 1104 includes a number of HD tracks, each of the HD tracks corresponding to a respective periodic waveform, in a conventional manner. In some approaches, significant features of the TBS pattern 1102 are retained, such as a servo frame structure having four servo bursts containing a number of servo stripes, where the servo stripes of adjacent servo bursts are written with alternating azimuthal angle. Other parameters of legacy servo patterns, such as the servo pattern height and other geometric dimensions, as well as the number of servo stripes per burst, may be modified as desired.

The detection of the periodic waveforms forming a HD pattern may be obtained by a conventional HD servo detector circuit that implements a complex algorithmic conversion, e.g., such as a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), etc.

FIG. 12 depicts a partial detailed view of a portion of a TBS pattern 1200 (e.g., a TBS frame of the TBS patterns shown in FIG. 10 or 11) according to an exemplary approach. As shown, a plurality of servo stripes 1202 together form a servo burst 1204, while corresponding pairs of servo bursts 1204 form servo sub-frames. Accordingly, the depicted TBS frame has four servo bursts 1204 and two servo sub-frames. In the present approach, the servo bursts 1204 included in the left servo sub-frame each have five servo stripes 1202, while the servo bursts 1204 included in the right servo sub-frame each have four servo stripes 1202. The servo stripes 1202 included in a given servo burst 1204 are oriented such that they have a same azimuthal slope represented by angle α. Moreover, corresponding pairs of servo bursts 1204 have opposing azimuthal slopes, thereby forming a chevron-type pattern. The height H and thickness t of the servo stripes 1202 may vary depending on the servo writer used to write the TBS pattern. According to an exemplary approach, which is in no way intended to limit the invention, the height H may be about 186 μm, and the angle α may be about 6°, while the thickness t is about 2.1 μm. Moreover, the spacing S between each of the servo stripes 1202 and/or the distance d between servo bursts 1204 having the same azimuthal slope may vary depending on the desired approach. According to an exemplary approach, which is in no way intended to limit the invention, the spacing S may be about 5 μm, while the distance d is about 100 μm. As described above, patterned transitions such as that shown in FIG. 12 allow for an estimate of the head lateral position to be determined by evaluating the relative timing of pulses generated by a servo reader reading the servo stripes 1202 of the servo burst 1204 as they are passed over the servo reader.

Referring again to FIG. 11, the HD pattern 1104 may include periodic waveforms written on adjacent tracks. For example, two periodic waveforms, characterized by two different spatial frequencies: low-frequency f₁ and high-frequency f₂, where f₂>f₁. However, a wider range of lateral head displacement is desired. Accordingly, a different configuration of the HD patterns may be used to avoid ambiguity in determining the lateral displacement.

FIG. 13 illustrates a graph 1300 plotting sample vs. amplitude of the TBS pattern 1200 of FIG. 12, detected as a servo readback signal 1302 during readback. A servo channel may decode the readback signal that is received from a servo reader of a magnetic tape head reading the TBS pattern 1200. For example, when a servo stripe 1202 of the TBS pattern 1200 passes across the servo sensor, a double pulse portion 1304 (having a positive peak and a negative peak) of the readback signal 1302 is generated, e.g., for purposes of an example see lateral dashed lines indicating how double pulse portions of the readback signal 1302 correspond to servo stripe read locations. Accordingly, two or more of such double pulse portions and timing associated therewith may be used in calculating lateral position (y-position) estimates.

In one approach, the servo channel may provide y-position estimates to a track-following control system, e.g., where such y-position estimates are calculated using Equation 1 below.

$\begin{matrix} {\overset{\hat{}}{y} = {\frac{d}{2{\tan(\alpha)}}\left( {\frac{1}{2} - \frac{\sum A_{i}}{\sum B_{i}}} \right)}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

As shown above, the lateral y-position estimate ŷ of Equation 1 may incorporate: the distance d, the azimuthal slope (angle α) of the servo stripes 1202, a measured time B_(i) between pairs of corresponding servo stripes with the same azimuth angle (e.g., parallel stripes //, or \\) from two different sub-frames, and a measured time A_(i) between pairs of corresponding servo stripes with opposite azimuth angle (e.g., stripes /\) from the same sub-frame.

For example, in the 5-5-4-4 pattern of FIG. 13, four measurements A_(i), i=0,1,2,3 and four measurements of B_(i), i=0,1,2,3 are performed per servo sub-frame of the TBS pattern 1200. In some approaches, the distance d is may be referred to as the “sub-frame length.”

A conventional servo detector may be used, with appropriate modifications as needed, to process servo signals in accordance with any of the approaches described herein. FIG. 14 shows a block diagram of a servo detector 1400 configured for the computation of the PES from a TBS pattern. The servo signal from the servo reader 1402 is detected using a servo signal detector 1404. The output of the detector 1404 is then processed by a PES computation unit 1410, which determines a PES estimate in a conventional manner, e.g., based on timing information from a servo signal.

As noted above, the quantity of data stored on a magnetic tape may be expanded by increasing the number of data tracks across the tape. More tracks are made possible by reducing feature sizes of the readers and writers, such as by using thin-film fabrication techniques and magnetoresistive (MR) sensors. However, for various reasons, the feature sizes of readers and writers cannot be arbitrarily reduced. Factors such as tape skew and tape lateral expansion and contraction must be dealt with to provide acceptable readback signals.

With regards to tape skew, and by way of example, FIGS. 15A-15D represent the effect of tape skew on reader position relative thereto. FIG. 15A shows a head 1500 relative to the tape 1502, where the tape has a nominal width and is oriented about parallel with the intended direction of tape travel 1501. As shown, the readers 1504 are aligned with the data tracks 1506 on the tape 1502. However, FIG. 15B shows the effect of skew. As the tape 1502 is skewed, it shifts from an orientation about parallel with the intended direction of tape travel 1501 (e.g., as shown in FIG. 15A), to being oriented at a skewed angle θ_(skew) relative to the intended direction of tape travel 1501. As shown, the readers 1504 are now positioned along the boundary between the data tracks 1506. Depending on the extent and/or direction of skew, the extent of misalignment between the readers and their corresponding data tracks may vary. Moreover, it should be noted that the exaggerated extent of the tape skew illustrated in FIG. 15B is presented for by way of example only.

In various approaches, tape skew may be compensated for by laterally moving the array of transducers and/or tilting the array of transducers relative to the longitudinal axis of the array, thereby selectively altering the location of the transducers in the array relative to the tape. Such tilting is preferably affected by rotating the array, but pivoting may also be employed in some approaches.

In one approach to compensate for tape skew, the head may be moved orthogonal to an intended direction of tape travel in order to realign the transducers with the skewed data tracks. Smaller adjustments may be made to keep the project reader span aligned with tracks on the tape. Looking momentarily to FIG. 15C, the head 1500 has been moved (e.g., shifted) orthogonal to the intended direction of tape travel 1501 in order to realign the readers 1504 with the skewed data tracks 1506.

In another approach to compensate for tape skew, the head may be tilted to position the longitudinal axis of the array of transducers approximately orthogonal to the actual direction of tape travel 1503 in order to realign the transducers with the skewed data tracks. Smaller adjustments may be made to keep the project reader span aligned with tracks on the tape. Looking momentarily to FIG. 15D, the head 1500 has been moved (e.g., tilted) orthogonal to the actual direction of tape travel 1503 in order to realign the readers 1504 with the skewed data tracks 1506.

This process of shifting and/or tilting of the head to realign the transducers with the skewed data tracks is effective when only one module is used (one row of elements). However, magnetic tape systems generally have two or more modules, each containing a row of transducers, where the additional rows of transducers allow for read verify during the write process. One difficulty in implementing this scheme in a head having multiple arrays, such as for the head in FIGS. 2, 2C and 3-7, is that though one array of transducers may be properly aligned with the tracks, the other arrays of transducers may not. Conventional products having more than one module implement a bonding process to fix the modules relative to each other. Currently, this bonding process creates a single movable sub assembly that is attached to a track following actuator typically motivated by a voice coil system. In other words, when the lateral orientation is changed for two or more modules that are fixed relative to each other, it is not possible to keep all elements on track since the other rows of elements will move as well.

FIGS. 16A-16C represent the effect of TDS, namely tape lateral expansion and contraction, on reader position relative to the tape. FIG. 16A shows a head 1600 relative to the tape 1602, where the tape has a nominal width. As shown, the readers 1604 are aligned with the data tracks 1606 on the tape 1602. FIG. 16B shows the effect of tape lateral contraction. As shown, the outermost readers 1608 are positioned along the outer edges of the outer data tracks. FIG. 16C shows the effect of tape lateral expansion. As shown, the outermost readers 1608 are positioned along the inner edges of the outer data tracks. Because all of the readers 1604 have the same width, the readback signal level from each reader will normally be the same.

While skew and TDS affect a magnetic head's ability to accurately read data from the various tracks in a data band as described above, it also affects servo readers' ability to accurately derive timing and positional information from the servo bands. For instance, while active skew control may enable read while write verification that helps to ensure reliability and active TDS control to enable higher track density and hence increased capacity, both TDS and skew are measured using servo readers.

As previously mentioned, servo readers are positioned on a module in a tape drive and read servo patterns that bracket each data band during operation of the tape drive. In some situations, the skew of the magnetic tape relative to a magnetic head containing the module and the servo readers thereon is measured by comparing the distance travelled between the arrival of a di-bit pulse observed with the top servo reader from a given stripe in the servo pattern and the pulse from the corresponding stripe observed with the bottom servo reader.

For instance, referring momentarily to FIGS. 17A-17B, the effects of skew are illustrated in the context of servo bands on magnetic tapes 1700, 1720. Specifically looking to FIG. 17A, no skew is present, as the top and bottom servo readers 1702, 1704 both observe (e.g., read) the leading stripe 1706 of their respective servo bursts 1708, 1710 at the same time. In other words, because there is no delay in determining when the servo readers 1702, 1704 each observe the leading stripe 1706 of their respective servo bursts 1708, 1710, it can be determined that no skew is present. However, looking to FIG. 17B, skew is present as evidenced by the top and bottom servo readers 1702, 1704 each observing (e.g., reading) the leading stripe 1706 of their respective servo bursts 1708, 1710 at different times. This difference in time between when the top and bottom servo readers 1702, 1704 observing the given leading stripes 1706 may be computed, but the servo pattern can also be used to estimate the current velocity at which the magnetic tape 1720 is moving relative to the servo readers 1702, 1704. It follows that the distance x, which represents the skew, can be determined using x=v t. This skew may also be converted from a distance and represented as an angle β.

While top-bottom skew estimation may be used to estimate skew as shown in FIG. 17B, for this measurement to be accurate, the top and bottom servo readers should be aligned to the same cross-track position on the servo pattern. In other words, the top and bottom servo readers should have the same YPOS location along the cross-track direction, e.g., as would be appreciated by one skilled in the art after reading the present description. Accordingly, top-bottom skew estimation may produce errors in situations where the magnetic tape has expanded or contracted in the cross-track direction due to TDS issues. These TDS issues may be measured by calculating the difference between the YPOS value measured with the top servo reader and that of the bottom servo reader, a value that is also referred to herein as the servo band difference (SBD). Accordingly, an increase in SBD corresponds to a contraction along the width of the magnetic tape.

Changes in TDS or tape width can be measured by changes in SBD and can be actively compensated for by adjusting tape tension or by adjusting angular position of the magnetic head relative to the magnetic tape. In previous products, tape tension is used to conduct active TDS compensation. However, this procedure is limited in range and introduces additional problems, thereby introducing a need to adopt new TDS compensation schemes, e.g., such as tilt based TDS compensation. Hence, under nominal operating conditions, the SBD is non-zero, even when active TDS compensation is used.

While this may improve alignment of transducers with their respective tracks on the magnetic tape, the non-zero SBD has resulted in an error for the measurement of skew for previous implementations. This is primarily because the two servo readers are not positioned at the same YPOS location on each of the servo bands as a result of the non-zero SBD. For instance, referring momentarily now to FIGS. 17C-17D, the negative effects caused by implementing a non-zero SBD while relying on previous skew estimation procedures are illustrated in the context of servo bands on magnetic tapes 1730, 1740. Looking to the magnetic tapes 1730, 1740 illustrated in FIGS. 17C-17D respectively, it is apparent that the magnetic tapes 1730, 1740 are currently experiencing tape contraction as evidenced by the actual YPOS 1712, 1714 of each of the servo readers being outside the intended YPOS 1716, 1718 of the servo readers relative to their servo pattern/band.

Looking specifically now to FIG. 17C, because of the tape contraction currently experienced by the magnetic tape 1730, while the servo readers 1702, 1704 are not skewed with respect to the magnetic tape 1730 and the servo bands thereon, each of the servo readers 1702, 1704 have a different YPOS in their respective servo bands, thereby causing the system to incorrectly assume tape skew is being experienced, e.g., as evidenced by distance x. In other words, the angled orientation of the stripes in the servo bursts 1708, 1710 paired with the different YPOS of the servo readers 1702, 1704 in their respective servo bands cause the skew estimation to conclude that the magnetic head is skewed relative to the magnetic tape, which is clearly not the case. Conversely, looking to FIG. 17D, while skew is actually being experienced, e.g., as evidenced by angle β, the tape contraction and resulting YPOS differences of the servo readers 1702, 1704 causes the system to incorrectly assume that the magnetic head is properly oriented relative to the magnetic tape 1740.

It follows that previous implementations are prone to experiencing skew estimation errors when TDS issues are encountered. These skew estimation errors in turn cause magnetic head positioning issues, read and/or write errors, data corruption, data loss, performance delays, etc. In sharp contrast to the aforementioned shortcomings experienced by conventional applications, various ones of the embodiments included herein are able to accurately determine tape skew by correcting for the errors caused by TDS, e.g., as will be described in further detail below.

Looking now to FIGS. 18A-18B, magnetic tapes 1800, 1850 are illustrated in accordance with two specific embodiments. As an option, either of the present magnetic tapes 1800, 1850 may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. However, such magnetic tapes 1800, 1850 and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the magnetic tapes 1800, 1850 presented herein may be used in any desired environment. Thus FIGS. 18A-18B (and the other FIGS.) may be deemed to include any possible permutation.

As depicted in FIGS. 18A-18B, both tapes 1800, 1850 have experienced lateral tape contraction along the cross-track direction 1890 due to TDS. Thus, the actual YPOS 1802, 1804 of each of the servo readers in the corresponding servo bands 1806, 1808 have extended away from each other, ultimately being positioned outside the intended YPOS 1810, 1812 of the servo readers in the respective servo bands 1806, 1808. The amount by which the YPOS of the servo reader 1814 in the “top” servo band 1806 has shifted is represented by the distance y^(TOP), while the amount by which the YPOS of the servo reader 1816 in the “bottom” servo band 1808 has shifted is represented by the distance y^(BOTTOM). However, it should be noted that the use of “top” and “bottom” herein is in no way intended to limit the invention. Moreover, the distance between the intended YPOS 1810, 1812 of the servo readers in the respective servo bands 1806, 1808 represents the servo band distance SBD.

Servo readers 1814, 1816 are positioned towards opposite ends of a module (not shown) along the longitudinal axis (e.g., longitudinal length) thereof. This module which contains the servo readers 1814, 1816 may also include one or more arrays of read and/or write transducers and be located in a magnetic tape drive, e.g., according to any of the approaches included herein. It should also be noted that although it appears there are four of each servo reader 1814, 1816 in both of FIGS. 18A-18B, this is merely done in order to depict the same servo reader at four different instances in time. In other words, the pair of servo readers 1814, 1816 are illustrated in four different positions relative to the servo bands 1806, 1808 as each of the respective magnetic tapes 1800, 1850 pass over the magnetic head (not shown) on which the servo readers 1814, 1816 are implemented.

As noted above, the tape lateral contraction seen in FIGS. 18A-18B affects how skew may be determined using the servo bands 1806, 1808. For instance, previous implementations have experienced skew estimation errors in situations involving TDS issues, thereby subjecting the tape drive and overall system to significant issues, as well as processing and/or computing delays, e.g., as described above. However, the processes described in conjunction with FIGS. 18A-18B are able to significantly improve these conventional shortcomings.

Looking specifically now to FIG. 18A, as noted above, the magnetic tape 1800 has experienced lateral tape contraction. Thus, although the servo readers 1814, 1816 are oriented relative to the magnetic tape 1800 such that no actual skew is present, the difference in YPOS for each of the servo readers 1814, 1816 causes the signals received from the servo readers 1814, 1816 to indicate skew is present. Once again, previous implementations have been unable to identify that although the timing based information received from servo readers indicates skew is present, no skew is actually being experienced.

In sharp contrast, in some approaches, by comparing the timing based information received from two or more different servo bursts, an accurate skew determination may be made, regardless of whether TDS issues are currently at play. Utilizing timing based information from more than one different servo burst effectively allows for the TDS issues to be canceled out, thereby producing higher resolution in skew estimation. For instance, the lateral tape contraction and absence of skew seen in FIG. 18A results in each of the servo readers 1814, 1816 detecting a leading stripe 1818 of the respective servo bursts B, A at a different point in time, as well as a trailing stripe 1824 of the respective servo bursts B, A. As noted above, each of these four points in time are represented in FIG. 18A simultaneously, e.g., such that the effects of the TDS and/or skew are more easily apparent.

Information pertaining to each of the points in time that the servo readers 1814, 1816 detect the leading stripes 1818 and the trailing stripes 1824 is preferably collected and stored. For instance, a timestamp associated with each of the aforementioned points in time may be received, noted, captured, stored, etc. It follows that in some approaches it is desirable to determine and store a timestamp I^(TOP) _(B) corresponding to the upper servo reader 1814 detecting the leading stripe 1818 of servo burst B in servo band 1806, a timestamp t^(TOP) _(A) corresponding to the upper servo reader 1814 detecting the trailing stripe 1824 of servo burst A in servo band 1806, a timestamp t^(BOTTOM) _(B) corresponding to the bottom servo reader 1816 detecting the leading stripe 1818 of servo burst B in servo band 1808, and a timestamp t^(BOTTOM) _(A) corresponding to the bottom servo reader 1816 detecting the trailing stripe 1824 of servo burst A in servo band 1808.

These timestamps may thereby be used to determine whether skew is actually being experienced. Again, because this information is collected from servo stripes in more than one burst, it may be used to counteract any TDS issues that are apparent for the magnetic tape. In other words, by averaging the timing based information received from the servo readers in response to detecting stripes from adjacent bursts, any discrepancies between the measured and actual skew cancel each other out, thereby producing the actual skew.

Referring still to FIG. 18A, the skew determined using the timestamps corresponding to the servo readers 1814, 1816 detecting the leading stripe 1818 of servo burst B in each of the respective servo bands 1806, 1808 indicates that an offset b_((B)) is present. However, the skew determined using the timestamps corresponding to the servo readers 1814, 1816 detecting the trailing stripe 1824 of servo burst A in each of the respective servo bands 1806, 1808 indicates that an offset −b_((A)) is present. By combining (e.g., averaging) these two skews determined from two different sets of servo bursts, the equal and opposite skews counteract each other, thereby informing the system that skew is not actually being experienced. This corresponds with the fact that there is no skew present between the magnetic head holding the servo readers and the magnetic tape on which the servo bands are patterned. This desirably informs the system that the tape is experiencing TDS issues rather than skew.

Equation 2 below may be used in some approaches to actually combine the skews determined from two different sets of servo bursts.

$\begin{matrix} {b = {{\frac{1}{2}\left( {t_{B}^{TOP} - t_{B}^{BOTTOM}} \right)v} + {\frac{1}{2}\left( {t_{A}^{TOP} - t_{A}^{BOTTOM}} \right)v}}} & {{Equation}\mspace{14mu} 2} \end{matrix}$

Here “b” represents the true (e.g., accurate) skew as a measured distance, “v” represents the velocity of the magnetic tape, and the remainder of the variables correspond to the description provided above. In other words, timestamp t^(TOP) _(B) corresponds to an upper servo reader (e.g., see 1814) detecting the leading stripe of a first servo burst in a first servo band (e.g., see servo burst B in servo band 1806), timestamp t^(TOP) _(A) corresponds to the upper servo reader detecting the trailing stripe of a second servo burst in the first servo band (e.g., see servo burst A in servo band 1806), timestamp t^(BOTTOM) _(B) corresponds to a bottom servo reader (e.g., see 1816) detecting the leading stripe of the first servo burst in a second servo band (e.g., see servo burst B in servo band 1808), and timestamp t^(BOTTOM) _(A) corresponds to the bottom servo reader detecting the trailing stripe of a second servo burst in the second servo band (e.g., see servo burst A in servo band 1808). Thus, the value expressed by (t^(TOP) _(B)−t^(BOTTOM) _(B))_(V) is essentially equal to the skew amount b_((B)) seen in FIG. 18A, while the value expressed by (t^(TOP) _(A)−t^(BOTTOM) _(A))_(V) is essentially equal to the skew amount −b_((A)) seen in FIG. 18A. However, it should be noted that the variable b as seen in Equation 2 represents the true (e.g., accurate) skew as a measured distance, while the skew amounts b_((B)) and −b_((A)) as seen in FIG. 18A are different representations of skew, e.g., as would be appreciated by one skilled in the art after reading the present description.

The timestamps may include any desired type of information that may be utilized by the various approaches included herein, e.g., as would be appreciated by one skilled in the art after reading the present description. For instance, in some approaches, each timestamp may simply indicate a normalized point in time that the respective servo stripe was detected. It should also be noted that while some of the approaches herein describe that timing based information from two adjacent bursts is utilized to determine the actual skew experienced by a magnetic tape, this is in no way intended to limit the invention. For instance, in other approaches, timing based information may be collected from more than two servo bursts, bursts that are not directly adjacent each other, etc.

As noted above, Equation 2 is able to combine (e.g., average) the skew measured with each of the servo bursts in order to achieve an accurate determination of the amount of skew that is actually present. This achievement is comparable to inserting a symmetry axis between the bursts of each servo sub-frame and using the timing information gathered by the servo readers as they pass thereacross in order to determine the actual skew. This symmetry axis is not subject to TDS issues in view of the fact that it is preferably oriented along the direction in which the tape expands and contracts. As a result, any difference in the timing information gathered from servo readers positioned towards opposite ends of a magnetic head can be attributed entirely to tape skew rather than a combination of tape skew and TDS. While such a symmetry axis may be purely theoretical in some approaches, in other approaches a symmetry axis may actually be formed on the magnetic tape between the bursts of each servo sub-frame. In such approaches, it is preferred that each symmetry axis extends along the entire width of the magnetic tape.

While Equation 2 is able to utilize the timing based information gleaned from two different servo bursts to determine the actual skew being experienced, span errors (e.g., trajectory errors) may also be at play. Accordingly, in some approaches LPOS information may be utilized to scale the determined skew in order to account for these span errors. For instance, Equation 3 below may be used in some approaches to determine a correction factor for a skew estimation determined using Equation 2.

$\begin{matrix} {b^{\prime} = {b\left( \frac{S\; B\; D}{y^{T} - y^{B} + {S\; B\; D}} \right)}} & {{Equation}\mspace{14mu} 3} \end{matrix}$

Here “b”′ represents the scaled skew, “b” represents the measured skew, SBD represents the servo band distance, y^(T) represents the YPOS of the top servo reader (e.g., see y^(TOP) in FIG. 18A), and y^(B) represents the YPOS of the bottom servo reader (e.g., see y^(BOTTOM) in FIG. 18A). However, it should be noted that the terms “top” and “bottom” are in no way intended to limit the invention, as any other factors may be used to distinguish between the servo bands.

It follows that Equation 2 and Equation 3 may be used to determine the actual skew being experienced in a given tape drive during operation thereof. The results of implementing Equation 2 and Equation 3 may also be used to adjust an orientation of a magnetic head relative to the magnetic tape in an effort to counteract tape skew and/or TDS issues, e.g., as described in various ones of the approaches included herein.

Looking now to FIG. 18B, another tape drive environment is depicted in which the tape is skewed relative to the servo readers 1814, 1816 and the magnetic head on which they are positioned. The magnetic tape is also experiencing TDS issues as discussed above, thereby causing difficulties in determining the actual skew. For instance, the angled orientation of the stripes in servo burst B of the respective servo bands 1806, 1808 result in the skew going unnoticed based on the timing based information alone as evidenced by b_((B)) being equal to zero. However, the mirrored orientation of the stripes in servo burst A of the respective servo bands 1806, 1808 result in the skew being exaggerated based on the timing based information gleaned therefrom as evidenced by b_((A)) appearing larger than it actually is.

Despite these skew concerns caused by the TDS issues, if the timing based information from both bursts are combined (e.g., averaged), the difference in the skew measured for each of the bursts cancel out and the resulting value accurately represents the actual skew being experienced. This may be achieved in some approaches by implementing Equation 2. Moreover, Equation 3 may be implemented in order to correct any span errors that may be present.

It follows that some of the approaches herein are able to use skew estimation derived from timing information that is gathered from two or more different burst to achieve an accurate understanding of skew estimation, even in TDS environments. Again, skew error is removed due to the fact that the TDS effects experienced by the two or more bursts cancel each other out by effectively averaging them. This desirably results in achieving a higher resolution skew estimation, thereby improving performance of the system as a whole, reducing a number of read and write errors experienced, increasing performance efficiency, etc.

While utilizing Equation 2 and Equation 3 along with timing based information extracted from the servo tracks in the servo bands on a magnetic tape allows for an accurate skew determination to be made, the timing based information may be processed differently in other approaches. For instance, rather than applying Equation 2 above, Equation 4.1 may be used in some approaches to actually combine the skews determined from two different sets of servo bursts.

$\begin{matrix} {b = {{\left( {t_{B}^{TOP} - t_{B}^{BOTTOM}} \right)v} - {{\frac{1}{2}\left\lbrack {\left( {t_{B}^{TOP} - t_{A}^{TOP}} \right) - \left( {t_{B}^{BOTTOM} - t_{A}^{BOTTOM}} \right)} \right\rbrack}v}}} & {{Equation}\mspace{14mu} 4.1} \end{matrix}$

The variables presented in Equation 4.1 represent the same factors as those discussed above with respect to Equation 2. For instance, “b” represents the measured skew, “v” represents the velocity of the magnetic tape, timestamp t^(TOP) _(B) corresponds to an upper servo reader (e.g., see 1814) detecting the leading stripe of a first servo burst in a first servo band (e.g., see servo burst B in servo band 1806), timestamp t^(TOP) _(A) corresponds to the upper servo reader detecting the trailing stripe of a second servo burst in the first servo band (e.g., see servo burst A in servo band 1806), timestamp t^(BOTTOM) _(B) corresponds to a bottom servo reader (e.g., see 1816) detecting the leading stripe of the first servo burst in a second servo band (e.g., see servo burst B in servo band 1808), and timestamp t^(BOTTOM) _(A) corresponds to the bottom servo reader detecting the trailing stripe of a second servo burst in the second servo band (e.g., see servo burst A in servo band 1808).

However, Equation 4.1 may further be simplified using the factors depicted in FIG. 18C. Looking momentarily to FIG. 18C, a servo frame 1870 is depicted in accordance with an example which is in no way intended to limit the invention. For instance, while each of the servo sub-frames are depicted as only including two stripes, this is done for illustration purposes and the sub-frames may each include any desired number of servo stripes. As shown, the YPOS offset value “y” may be represented as follows.

$y = {\frac{1}{\tan(\alpha)}\frac{d}{2}\left( {\frac{1}{2} - \frac{\sum A_{i}}{\sum B_{i}}} \right)}$

This equation may further be simplified into the following equation.

${d\frac{\sum A_{i}}{\sum B_{i}}} = {\frac{d}{2} - {2y\;\tan\;(\alpha)}}$

With this information, Equation 4.1 may be simplified into Equation 4.2 as follows.

$\begin{matrix} {b = {{\left( {t_{B}^{TOP} - t_{B}^{BOTTOM}} \right)v} - {\frac{1}{2}\left\lbrack {{d\frac{\sum A_{i}^{TOP}}{\sum B_{i}^{TOP}}} - {d\frac{\sum A_{i}^{BOTTOM}}{\sum B_{i}^{BOTTOM}}}} \right\rbrack}}} & {{Equation}\mspace{14mu} 4.2} \end{matrix}$

Furthermore, Equation 4.2 may be simplified again to arrive at Equation 4.3 or equivalently Equation 4.4 as seen below. b=(t _(B) ^(TOP) −T _(B) ^(BOTTOM))v+tan(α)[y ^(TOP) −y ^(BOTTOM)]   Equation 4.3 b=b _(B)+tan(α)[y ^(TOP) −y ^(BOTTOM)]   Equation 4.4

Here, “b” represents the true (e.g., accurate) skew as a measured distance, “b_(B)” represents the skew determined using the timing information gathered from the servo readers detecting a leading stripe of corresponding servo bursts (e.g., see b_(B) in FIGS. 18A-18B), y^(TOP) represents the YPOS of the top servo reader (e.g., see y^(TOP) in FIG. 18A), and y^(BOTTOM) represents the YPOS of the bottom servo reader (e.g., see y^(BOTTOM) in FIG. 18A). Moreover, “α” represents the angle measured between: a given reference line (e.g., see dashed line 1899 of FIG. 18C,) that extends along the cross-track direction, and a given one of the stripes in one of the servo bands. As a result, an accurate skew determination may be made using the timing based information gathered from a single servo burst in each servo band, thereby increasing the rate at which skew can be determined, reducing processing delays, etc. Moreover, the skew determined using Equation 4.4 is not subject to affects caused by TDS issues.

While Equation 4.4 is able to utilize the timing based information gleaned from a servo burst in each of the servo bands to determine the actual skew being experienced, span errors (e.g., trajectory errors) may also be at play. Accordingly, in some approaches additional LPOS information may be utilized to scale the determined skew in order to account for these span errors. For instance, Equation 5 below may be used in some approaches to determine a correction factor for a skew estimation determined using Equation 4.4.

$\begin{matrix} {b^{\prime} = {b\left( \frac{y^{T} - y^{B} + {S\; B\; D}}{S\; B\; D} \right)}^{- 1}} & {{Equation}\mspace{14mu} 5} \end{matrix}$

As noted above with respect to Equation 3, “b” represents the scaled skew, “b” represents the measured skew, SBD represents the servo band distance, y^(T) represents the YPOS of the top servo reader (e.g., see y^(TOP) in FIG. 18A), and y^(B) represents the YPOS of the bottom servo reader (e.g., see y^(BOTTOM) in FIG. 18A). However, it should be noted that the terms “top” and “bottom” are in no way intended to limit the invention, as any other factors may be used to distinguish between the servo bands.

It follows that Equation 4.4 and Equation 5 may be used to determine the actual skew being experienced in a given tape drive during operation thereof. The results of implementing Equation 4.4 and Equation 5 may also be used to adjust an orientation of a magnetic head relative to the magnetic tape in an effort to counteract tape skew and/or TDS issues, e.g., as described in various ones of the approaches included herein.

Referring now to FIG. 19, a flowchart of a computer-implemented method 1900 for determining skew of a magnetic tape in a TDS environment is shown according to one embodiment. In other words, the various processes included in method 1900 may be used to make an accurate determination of the skew experienced in a given tape drive despite the presence of TDS issues. The method 1900 may be performed in accordance with the present invention in any of the environments depicted in FIGS. 1-18C, among others, in various embodiments. Of course, more or less operations than those specifically described in FIG. 19 may be included in method 1900, as would be understood by one of skill in the art upon reading the present descriptions.

Each of the steps of the method 1900 may be performed by any suitable component of the operating environment using known techniques and/or techniques that would become readily apparent to one skilled in the art upon reading the present disclosure. For example, in various embodiments, the method 1900 may be partially or entirely performed by a controller, a processor, etc., or some other device having one or more processors therein. The processor, e.g., processing circuit(s), chip(s), and/or module(s) implemented in hardware and/or software, and preferably having at least one hardware component may be utilized in any device to perform one or more steps of the method 1900. Illustrative processors include, but are not limited to, a central processing unit (CPU), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc., combinations thereof, or any other suitable computing device known in the art.

As shown in FIG. 19, operation 1902 of method 1900 includes receiving a first timestamp t¹ _(B) in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape. In some approaches the first timestamp t¹ _(B) may correspond to a servo reader detecting a leading stripe of the given servo burst. For instance, the first timestamp received in operation 1902 may be the timestamp t^(TOP) _(B) from FIG. 18A which corresponds to a servo reader detecting the leading stripe of a first servo burst in a first servo band. However, referring still to FIG. 19, in other approaches the first timestamp t¹ _(B) may correspond to the servo reader detecting a trailing stripe of the given servo burst, a middle stripe of the given servo burst, etc. The first timestamp t¹ _(B) may be received directly from the servo reader that detected the corresponding stripe in some approaches, while in other approaches the first timestamp t¹ _(B) may be received from a controller, a tape drive, memory, etc. Similarly, any of the timestamps received in method 1900 may be received directly from a servo reader, a controller, a tape drive, memory, etc.

Operation 1904 of method 1900 includes receiving a second timestamp t² _(B) in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape, while operation 1906 includes receiving a third timestamp t¹ _(A) in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape. In one approach, the second timestamp t² _(B) may correspond to a servo reader detecting a leading stripe of the given servo burst. For instance, the second timestamp received in operation 1904 may be the timestamp t^(BOTTOM) _(B) from FIG. 18A which corresponds to a servo reader detecting the leading stripe of a first servo burst in a second servo band. According to another approach, the third timestamp t¹ _(A) may correspond to a servo reader detecting a trailing stripe of the given servo burst. For instance, the second timestamp received in operation 1906 may be the timestamp t^(TOP) _(A) from FIG. 18A which corresponds to a servo reader detecting the trailing stripe of a second servo burst in a first servo band. However, referring still to FIG. 19, in other approaches the second timestamp t² _(B) and/or the third timestamp t¹ _(A) may correspond to the servo reader detecting a trailing stripe of the given servo burst, a middle stripe of the given servo burst, etc.

Method 1900 further includes receiving a fourth timestamp t² _(A) in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape. See operation 1908. In one approach, the fourth timestamp t² _(A) may correspond to a servo reader detecting a trailing stripe of the given servo burst. For instance, the fourth timestamp received in operation 1908 may be the timestamp t^(BOTTOM) _(A) from FIG. 18A which corresponds to a servo reader detecting the trailing stripe of a second servo burst in a second servo band. However, referring still to FIG. 19, in other approaches the fourth timestamp t² _(A) may correspond to the servo reader detecting a trailing stripe of the given servo burst, a middle stripe of the given servo burst, etc.

It should be noted that any one or more of the first, second, third, and fourth timestamps may include any desired type of information that may be utilized by the various approaches included herein. For instance, in some approaches, each timestamp may simply indicate a normalized point in time that the respective servo stripe was detected. In other approaches, the timestamp may include metadata that can be used to determine a current lateral position (LPOS) on the tape, e.g., as would be appreciated by one skilled in the art after reading the present description. It should also be noted that while some of the approaches herein describe that timing based information from two adjacent bursts is utilized to determine the actual skew experienced by a magnetic tape, this is in no way intended to limit the invention. For instance, in other approaches, timing based information may be collected from more than two servo bursts, bursts that are not directly adjacent each other, etc.

With continued reference to FIG. 19, operation 1910 includes using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape. As noted above, by determining whether skew is present using timing based information gathered from more than one servo burst, it may be used to counteract any TDS issues that are apparent for the magnetic tape. In other words, by averaging the timing based information received from the servo readers in response to detecting stripes from adjacent bursts, any discrepancies between the measured and actual skew cancel each other out, thereby producing the actual skew.

The manner in which the timestamps are used to actually determine the skew may vary depending on the desired approach. For instance, in some approaches the first, second, third, and fourth timestamps may be used in conjunction with Equation 2 above in order to determine the skew of the magnetic tape. In other approaches, the first, second, third, and fourth timestamps may be used in conjunction with Equation 4.4 above in order to determine the skew of the magnetic tape. In still other approaches, the first, second, third, and fourth timestamps may be used in conjunction with timing based information which corresponds to one or more servo readers detecting a symmetry axis extending along the cross-track direction and positioned between servo bursts in the same sub-frame. It follows that the first, second, third, and fourth timestamps may be used to determine the skew of the magnetic tape differently depending on the desired approach. For instance, the amount and/or type of timing based information included in each of the timestamps may have an impact on what processes are implemented in order to determine the skew.

While skew is accurately determined in operation 1910 by utilizing the timing based information gleaned from more than one servo burst, span errors (e.g., trajectory errors) may also be at play. Accordingly, in some approaches additional LPOS information may be utilized to scale skew that has been determined in order to account for these span errors. Accordingly, optional operation 1912 includes scaling the determined skew of the magnetic tape to account for any span errors. This scaling process may utilize a position of the first servo sub-frame along the cross-track direction of the magnetic tape, as well as a position of the second servo sub-frame along the cross-track direction of the magnetic tape. In other words, the scaling process may incorporate the LPOS of each of the servo bands.

According to one approach, the determined skew of the magnetic tape may be used in conjunction with LPOS information and Equation 3 in order to determine a correction factor for the determined skew. According to another approach, the determined skew of the magnetic tape may be used in conjunction with LPOS information and Equation 5 in order to determine a correction factor for the determined skew.

The scaled skew of the magnetic tape may thereafter be used in a tape drive to reduce skew as well as improve read and/or write performance. Accordingly, operation 1914 includes sending the scaled skew of the magnetic tape to the tape drive in which the magnetic tape is being processed. This scaled skew may thereby be used by: actuators to tilt a magnetic head to compensate for skew, elements that control the operating tension of the magnetic tape, environmental control components (e.g., a humidifier), etc., such that skew may be counteracted and performance of the system as a whole improves.

From operation 1914, method 1900 may end. Although method 1900 may end, the processes included therein may be repeated any desired number of times, and at any desired frequency. Thus, method 1900 may be performed periodically at a predetermined period, repeatedly, in response to a predetermined condition being met (e.g., such as experiencing a read and/or write error), in response to receiving a request to determine whether skew is present, etc. It should also be noted that method 1900 may be performed in the background such that the tape drive is able to function properly without experiencing any processing and/or performance delays.

It follows that the processes included in method 1900 are able to accurately determine skew despite TDS issues being experienced. As a result, some of the approaches herein are able to improve track following performance, thereby increasing system throughput, reducing errors, improving achievable track density limits, etc.

It should also be noted that although skew is represented in various ones of the embodiments included herein as a distance (e.g., see b in FIGS. 18A-18C), this measurement may be converted such that it is represented as an angle. This conversion from a distance to an angle may be performed using any processes which would be apparent to one skilled in the art after reading the present description.

The present invention may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be accomplished as one step, executed concurrently, substantially concurrently, in a partially or wholly temporally overlapping manner, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

Moreover, a system according to various embodiments may include a processor and logic integrated with and/or executable by the processor, the logic being configured to perform one or more of the process steps recited herein. The processor may be of any configuration as described herein, such as a discrete processor or a processing circuit that includes many components such as processing hardware, memory, I/O interfaces, etc. By integrated with, what is meant is that the processor has logic embedded therewith as hardware logic, such as an application specific integrated circuit (ASIC), a FPGA, etc. By executable by the processor, what is meant is that the logic is hardware logic; software logic such as firmware, part of an operating system, part of an application program; etc., or some combination of hardware and software logic that is accessible by the processor and configured to cause the processor to perform some functionality upon execution by the processor. Software logic may be stored on local and/or remote memory of any memory type, as known in the art. Any processor known in the art may be used, such as a software processor module and/or a hardware processor such as an ASIC, a FPGA, a central processing unit (CPU), an integrated circuit (IC), a graphics processing unit (GPU), etc.

It will be clear that the various features of the foregoing systems and/or methodologies may be combined in any way, creating a plurality of combinations from the descriptions presented above.

It will be further appreciated that embodiments of the present invention may be provided in the form of a service deployed on behalf of a customer.

The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, embodiments, and/or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in any combination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the art upon reading the instant descriptions should also be considered within the scope of this disclosure.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A computer-implemented method for determining skew of a magnetic tape in a tape dimensional stability (TDS) environment, the computer-implemented method comprising: receiving a first timestamp t¹ _(B) in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape; receiving a second timestamp t² _(B) in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape; receiving a third timestamp t¹ _(A) in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape; receiving a fourth timestamp t² _(A) in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape; and using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape, wherein the first and third servo bursts are in a same first servo sub-frame, wherein the second and fourth servo bursts are in a same second servo sub-frame.
 2. The computer-implemented method as recited in claim 1, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: $b = {{\frac{1}{2}\left( {t_{B}^{1} - t_{B}^{2}} \right)v} + {\frac{1}{2}\left( {t_{A}^{1} - t_{A}^{2}} \right)v}}$ wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape.
 3. The computer-implemented method as recited in claim 1, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: b=(t _(B) ¹ −t _(B) ²)v+tan(α)[y ¹ −y ²] wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape, wherein a represents an angle measured between a reference line extending along a cross-track direction, and a given one of the stripes in one of the first and second servo sub-frames, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 4. The computer-implemented method as recited in claim 1, wherein the first servo sub-frame is in a first servo band that extends along a longitudinal length of the magnetic tape, wherein the second servo sub-frame is in a second servo band that extends along the longitudinal length of the magnetic tape, wherein the first and second servo bands are positioned on opposite sides of a data band along a cross-track direction of the magnetic tape, wherein the first and second servo readers are positioned towards opposite ends of a module along a longitudinal axis thereof.
 5. The computer-implemented method as recited in claim 1, comprising: scaling the determined skew of the magnetic tape using: a position of the first servo sub-frame along a cross-track direction of the magnetic tape, and a position of the second servo sub-frame along the cross-track direction of the magnetic tape.
 6. The computer-implemented method as recited in claim 5, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{y^{1} - y^{2} + {S\; B\; D}}{S\; B\; D} \right)}^{- 1}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 7. The computer-implemented method as recited in claim 5, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{S\; B\; D}{y^{1} - y^{2} + {S\; B\; D}} \right)}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 8. A computer program product for determining skew of a magnetic tape in a tape dimensional stability (TDS) environment, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions readable and/or executable by a processor to cause the processor to: receive, by the processor, a first timestamp t¹ _(B) in response to a first servo reader detecting a stripe of a first servo burst on the magnetic tape; receive, by the processor, a second timestamp t² _(B) in response to a second servo reader detecting a stripe of a second servo burst on the magnetic tape; receive, by the processor, a third timestamp OA in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape; receive, by the processor, a fourth timestamp t² _(A) in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape; and use, by the processor, the first, second, third, and fourth timestamps to determine the skew of the magnetic tape, wherein the first and third servo bursts are in a same first servo sub-frame, wherein the second and fourth servo bursts are in a same second servo sub-frame.
 9. The computer program product as recited in claim 8, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: $b = {{\frac{1}{2}\left( {t_{B}^{1} - t_{B}^{2}} \right)v} + {\frac{1}{2}\left( {t_{A}^{1} - t_{A}^{2}} \right)v}}$ wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape.
 10. The computer program product as recited in claim 8, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: $b = {{\left( {t_{B}^{1} - t_{B}^{2}} \right)v} - {{\frac{1}{2}\left\lbrack {\left( {t_{B}^{1} - t_{A}^{1}} \right) - \left( {t_{B}^{2} - t_{A}^{2}} \right)} \right\rbrack}v}}$ wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape.
 11. The computer program product as recited in claim 8, wherein the first servo sub-frame is in a first servo band that extends along a longitudinal length of the magnetic tape, wherein the second servo sub-frame is in a second servo band that extends along the longitudinal length of the magnetic tape, wherein the first and second servo bands are positioned on opposite sides of a data band along a cross-track direction of the magnetic tape, wherein the first and second servo readers are positioned towards opposite ends of a module along a longitudinal axis thereof.
 12. The computer program product as recited in claim 8, wherein the program instructions are readable and/or executable by the processor to cause the processor to: scale, by the processor, the determined skew of the magnetic tape using: a position of the first servo sub-frame along a cross-track direction of the magnetic tape, and a position of the second servo sub-frame along the cross-track direction of the magnetic tape.
 13. The computer program product as recited in claim 12, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{y^{1} - y^{2} + {S\; B\; D}}{S\; B\; D} \right)}^{- 1}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 14. The computer program product as recited in claim 12, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{S\; B\; D}{y^{1} - y^{2} + {S\; B\; D}} \right)}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 15. A system, comprising: a tape drive having at least two servo readers; a processor; and logic integrated with the processor, executable by the processor, or integrated with and executable by the processor, the logic being configured to: receive, by the processor, a first timestamp t¹ _(B) in response to a first of the servo readers detecting a stripe of a first servo burst on a magnetic tape; receive, by the processor, a second timestamp t² _(B) in response to a second of the servo readers detecting a stripe of a second servo burst on the magnetic tape; receive, by the processor, a third timestamp OA in response to the first servo reader detecting a stripe of a third servo burst on the magnetic tape; receive, by the processor, a fourth timestamp t² _(A) in response to the second servo reader detecting a stripe of a fourth servo burst on the magnetic tape; and use, by the processor, the first, second, third, and fourth timestamps to determine the skew of the magnetic tape, wherein the first and third servo bursts are in a same first servo sub-frame, wherein the second and fourth servo bursts are in a same second servo sub-frame.
 16. The system as recited in claim 15, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: $b = {{\frac{1}{2}\left( {t_{B}^{1} - t_{B}^{2}} \right)v} + {\frac{1}{2}\left( {t_{A}^{1} - t_{A}^{2}} \right)v}}$ wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape.
 17. The system as recited in claim 15, wherein using the first, second, third, and fourth timestamps to determine the skew of the magnetic tape includes solving the following equation: $b = {{\left( {t_{B}^{1} - t_{B}^{2}} \right)v} - {{\frac{1}{2}\left\lbrack {\left( {t_{B}^{1} - t_{A}^{1}} \right) - \left( {t_{B}^{2} - t_{A}^{2}} \right)} \right\rbrack}v}}$ wherein b represents the determined skew of the magnetic tape, wherein v represents a velocity of the magnetic tape.
 18. The system as recited in claim 15, wherein the logic is configured to: scale, by the processor, the determined skew of the magnetic tape using: a position of the first servo sub-frame along a cross-track direction of the magnetic tape, and a position of the second servo sub-frame along the cross-track direction of the magnetic tape.
 19. The system as recited in claim 18, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{y^{1} - y^{2} + {S\; B\; D}}{S\; B\; D} \right)}^{- 1}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents a lateral position of the first servo reader relative to the first servo sub-frame along the cross-track direction, wherein y² represents a lateral position of the second servo reader relative to the second servo sub-frame along the cross-track direction.
 20. The system as recited in claim 18, wherein scaling the determined skew of the magnetic tape includes solving the following equation: $b^{\prime} = {b\left( \frac{S\; B\; D}{y^{1} - y^{2} + {S\; B\; D}} \right)}$ wherein b′ represents the scaled skew of the magnetic tape, wherein b represents the determined skew of the magnetic tape, wherein SBD represents a servo band distance, wherein y¹ represents the position of the first servo reader in the first servo sub-frame along the cross-track direction, wherein y² represents the position of the second servo reader in the second servo sub-frame along the cross-track direction. 